Bouncing or Merging in Drop-Bath Impacts

Abstract

When a liquid drop impacts a liquid surface, it deforms the surface, trapping a thin layer of air between drop and bath. The drop then either bounces off the bath or, if the gas film ruptures, merges with it. We present a model for drop-bath impacts that can accurately predict whether a drop will bounce or merge, and compare it to theory-driven experiments. The model incorporates a lubrication description of the deforming trapped gas film, which accounts for its significant curvature, within a finite-element framework. Capturing this outcome requires micro- and nano-scale physics: van der Waals-driven disjoining pressure and gas-kinetic effects that depend on the mean free path of the gas. Using only independently measured material parameters, we obtain the first quantitative agreement with experiment for the critical impact speed separating bounce from merge. We explore the behaviour of drop-bath impacts at low ambient air pressure, and then investigate the bounce-merge threshold for impacts onto shallow baths and thin films, finding a bounce-merge-bounce-merge transition as the bath depth decreases.